We recognized two fundamental lithostratigraphic units at Site 1178 (Fig. F1; Table T3). Unit I represents the slope-apron facies, and Unit II consists of accreted sediments. Both are divided into three subunits. Interpretations of the lithostratigraphy are hampered by complexities in biostratigraphy and deformation by thrust faulting. Another noteworthy aspect of this site is that nearly all of Unit I has been subjected to significant amounts of displacement along a submarine slide surface. Strata below the dislocation surface are almost certainly part of the Nankai Trough accretionary prism. We recognize striking similarities between their facies associations and those of the axial to outer trench-wedge environments of the Nankai Trough, as described at Sites 1173 and 1174. The widespread occurrence of such deformation features as incipient cleavage, spaced fractures, steep bedding dips, and brecciation supports the notion of frontal accretion (see "Structural Geology").
Subunit IA is Quaternary to Pliocene in age and extends from the seafloor to a depth of 94.40 mbsf (Section 190-1178A-12X-1, 0 cm). This unit consists predominantly of nannofossil-rich hemipelagic mud (silty clay to clayey silt) with lesser amounts of sandy mud (sand-silt-clay). The hemipelagic mud and sandy mud are gray, greenish gray, or greenish brown in color. Internal structures, ranging from homogeneous to layered with sand laminae or mottled green clay-rich laminae are a result of bioturbation. The composition includes abundant clay minerals and biogenic debris together with lesser amounts of volcanic glass and lithic fragments (see "Site 1178 Smear Slides"). Nannofossils, diatoms, and sponge spicules are common in this unit. Sandy intervals contain a typical assortment of such siliciclastic grains as quartz, plagioclase, pyroxene, and lithic fragments.
Volcanic ash beds in Subunit IA vary in thickness from <1 cm to medium bedded (19 cm). The thickest ash deposit is in Core 190-1178A-3H (14.12 mbsf). Ash layers typically have sharp, plane-parallel to irregular lower contacts and gradational upper contacts (Fig. F2). Color variations range from pale gray to dark gray, white with dark grains (salt and pepper), brown, pink, and greenish gray. Grain size varies from lapilli to fine ash; a grain-size range of 0.1-2.0 mm is most common. The volcaniclastic sediment is composed primarily of fresh glass shards or pumice, plus variable amounts of nannofossil-rich silty clay, lithic clasts, and crystals of quartz, plagioclase, and pyroxene (see "Site 1178 Smear Slides").
Intervals of disturbed stratification are common within Subunit IA (Fig. F3). Although some of the disturbance may be due to coring or drilling, we attribute most of the chaotic and folded bedding to downslope movement of poorly lithified sediment. The following intervals contain the most obvious examples of disturbed stratification: intervals 190-1178A-4H-1, 125 cm, to 4H-6, 139 cm (23.15-29.79 mbsf); 5H-2, 50 cm, to 5H-4, 130 cm (33.40-37.20 mbsf); 6H-3, 50 cm, to 7H-4, 80 cm (44.40-55.70 mbsf); and 8H-2, 35 cm, to 8H-5, 120 cm (59.35-64.70 mbsf). Additional zones of deformation may have escaped our recognition because of homogeneous lithology.
The overall facies character of Subunit IA is consistent with slope-apron sedimentation by way of hemipelagic settling of nannofossil-rich mud, mudflows, and occasional air falls of volcanic ash. A relatively high concentration of calcareous nannofossils in the hemipelagic mud indicates that deposition occurred above the CCD. Remobilization of the slope sediment by large-scale gravitational failure probably caused the internal stratal disruption of Subunit IA.
Subunit IB is Pliocene in age and extends from 94.40 mbsf (Section 190-1178A-12X-1, 0 cm) to 127.00 mbsf (Section 15X-3, 80 cm). Lithification of the fine-grained strata is more advanced than in Subunit IA, but the hemipelagic deposits are also interstratified with thin beds of silty sand. These turbidites provide the most distinguishing characteristic of Subunit IB. They display sharp bases, plane-parallel laminae, and ripple cross-laminae. In addition, many of the intervening muddy intervals contain appreciable quantities of coarse silt and fine sand, some of which may have been dispersed by the coring and core-splitting process. Beds of mud-supported gravel (Fig. F4) are present in Sections 190-1178A-13X-1 and 13X-2, and 14X-2. There is also one unusual bed of volcanic lapilli and gravel-sized clasts of mudstone in Section 14X-3 (Fig. F5).
Stratal disruption is also commonplace within Subunit IB. The most clearly defined intervals of soft-sediment deformation are 190-1178A-12X-3, 30 cm, to 12X-6, 55 cm (97.61-102.36 mbsf), and 14X-6, 25 cm, to 15X-3, 80 cm (121.35-127.00 mbsf). Deposition of Subunit IB occurred during a time interval in which fine-grained turbidity currents were relatively frequent. Within a slope-apron environment, the most likely mechanism for periodic turbidite influx is overbank deposition associated with a nearby submarine canyon system.
Subunit IC is Pliocene to late Miocene in age and extends from 127.00 mbsf (interval 190-1178A-15X-3, 80 cm) to 199.20 mbsf (23X-1, 0 cm). The base of Subunit IC coincides with a prominent seismic reflector; reflector orientation and lateral continuity change significantly below the subunit boundary. This dislocation surface is parallel to the present-day seafloor, so it is probably a failure plane at the base of a large submarine slump or slide.
The most common lithology of Subunit IC is greenish gray hemipelagic mud (clayey silt), with variable amounts of intermixed sand. The mud and sandy mud is fractured, and sedimentary brecciation occurs throughout the subunit. Deformation within the lowermost interval (Sections 190-1178A-22X-4 to 22X-CC) is particularly intense, reducing most of the core to slurry. Angular clasts of green mud and mottled, pale brown patches are scattered through the subunit. There are also rare beds of volcanic ash and volcanic lapilli mixed with gravel-sized mud clasts. The original depositional setting for Subunit IC was probably an inclined slope apron without turbidite influx, similar to the environmental conditions that existed during deposition of Subunit IA.
Subunit IIA is late Miocene in age and extends from 199.20 mbsf (Section 190-1178A-23X-1, 0 cm) to 411.00 mbsf (Section 190-1178B-4R-1, 140 cm). The most characteristic feature of this subunit is its preponderance of thin-bedded, poorly lithified turbidites, which range in texture from sand to sandy silt and silt. Most such beds are 1-10 cm in thickness with sharp bases and gradational tops. Internal sedimentary structures include plane-parallel laminae and ripple cross-laminae (Fig. F6). Normal size grading is common. In a typical 1.5-m section of core, there may be three or four sand-silt beds interlayered with hemipelagic mud, mudstone, and muddy turbidites. There are also scattered light brown carbonate-cemented mudstones.
Thicker beds of poorly lithified sand and muddy sand are present in Cores 190-1178A-30X, 40X, and 42X and in Core 190-1178B-4R. These deposits are as thick as 150 cm, poorly sorted, and normally graded; some contain dispersed clasts of mudstone. Fragments of woody organic matter are also common (Fig. F7). Common grain constituents include quartz, plagioclase, volcanic lithic fragments, and sedimentary to metasedimentary rock fragments. The base of Subunit IIA coincides with the deepest occurrence of thick-bedded sand (interval 190-1178B-4R-1, 0-140 cm).
The facies character of Subunit IIA is nearly identical to the sand-rich axial trench-wedge facies that was cored at Sites 808 and 1174. Frequent turbidity currents affected the depositional environment, and rates of sedimentation were high. These environmental conditions, together with a low content of calcium carbonate, are consistent with deposition near the base of a slope. Subsequent accretion to the Nankai Trench slope occurred by frontal offscraping. The base of Subunit IIA is close to the position of a thrust fault within the prism, as described in "Structural Geology".
Subunit IIB is late Miocene in age and extends from 411.00 (Section 190-1178B-4R-1, 140 cm) to 563.95 mbsf (Section 190-1178B-20R-1, 35 cm). Most of this subunit is composed of gray hemipelagic mudstone (silty claystone to clayey siltstone) with local mottling and bioturbation. As subsidiary lithologies, there are sporadic interbeds of silt, sandy silt to silty sand, sandy mud (sand-silt-clay), and bands of greenish gray mudstone. Stratification is typically inclined, and many intervals of mudstone display a well-developed fissility or incipient cleavage (see "Structural Geology"). Locally, the incipient cleavage is oblique to the stratification. Inclined fractures are also common, and thrust faults are evident at ~525 and ~550 mbsf (Fig. F1).
The lithofacies character of Subunit IIB is virtually identical to what we observed in the outer trench-wedge facies at Sites 1173 and 1174, as well as in the correlative facies unit at Site 808 (Shipboard Scientific Party, 1991). Agents of deposition included hemipelagic settling, distal excursions of sand-silt turbidity currents onto the outer margin of the Nankai Trough, and settling of mud from the entrained layers of turbidity currents. The environmental change from Subunit IIB into IIA was gradual (i.e., an incremental increase in sandy turbidites upsection); therefore, the subunit boundary is somewhat arbitrary. Subsequent accretion to the Nankai Trench slope probably was responsible for the widespread deformation of strata within Subunit IIB.
Most strata within Unit II are strongly deformed; spaced fractures, fissility, incipient cleavage, and steep bedding dips are common. Without observing changes in lithofacies or stratigraphic age, it is difficult to identify the location or amount of displacement on individual faults within the accretionary prism. A pronounced shift in lithofacies does occur, however, at 563.95 mbsf (Section 190-1178B-20R-1, 35 cm). Below that depth, facies relations once again are consistent with a sand-rich axial trench-wedge environment, as opposed to an outer trench-wedge environment. Beds of unlithified sand and silty sand increase in both number and thickness within Subunit IIC. The similarity of these strata to the deposits of Subunit IIA is so strong that we interpret the facies change to be the result of repetition along one of the imbricate faults of the accretionary prism. Structural criteria place such a fault at ~550 mbsf (Fig. F1).
The results of X-ray diffraction (XRD) analysis of bulk-sediment samples from Site 1178 are shown in Figure F8 (see Tables T4 and T5 for data). Stratigraphic trends are nondescript except for a reduction in calcite content from Units I and II. The higher content of calcite within Unit I is consistent with deposition of the slope apron above the CCD. Depletion of calcite within the accreted trench-wedge deposits of Unit II is consistent with deposition of those strata below the CCD. Relative mineral abundances within Unit II are scattered and inconsistent because of the interstratification of hemipelagic mud, muddy turbidites, and sand-silt turbidites, as well as the repetition of facies by thrust faults.